This invention relates generally to electrostatic precipitators for air pollution control and, more specifically, concerns the electrical control of electrostatic precipitators.
Continuous emphasis on environmental quality has resulted in increasingly strenuous regulatory controls on industrial emissions. One technique which has proven highly effective in controlling air pollution has been the removal of undesirable particulate matter from a gas stream by electrostatic precipitation. An electrostatic precipitator is an air pollution control device designed to electrically charge and collect particulates generated from industrial processes such as those occurring in cement plants, pulp and paper mills and utilities. Particulate laden gas flows through the precipitator where the particulate is negatively charged. These negatively charged particles are attracted to, and collected by, positively charged metal plates. The cleaned process gas may then be further processed or safely discharged to the atmosphere.
To maximize the particulate collection, a precipitator should be operated at the highest practical energy level to increase both the particle charge and collection capabilities of the system. Concurrently, there is a level above which "sparking" (i.e., a temporary short which creates a conductive gas path) occurs in the system. Left uncontrolled, this sparking can damage the precipitator and control system. The key to maximizing the efficiency of an electrostatic precipitator is to operate at the highest energy level possible.
Ideally, the electrostatic precipitator should constantly operate at its point of greatest efficiency. Unfortunately, the conditions, such as temperature, combustion rate, and the chemical composition of the particulate being collected, under which an electrostatic precipitator operates are constantly changing. This complicates the calculation of parameters critical to a precipitator's operation. This is particularly true of the current limiting reactor (CLR) which controls and limits the current entering the precipitator and matches the precipitator load to the line to allow for maximum power transfer to the precipitator.
The current limiting reactor (CLR) has two main functions. The first is to shape the voltage and current wave forms that appear in the precipitator for maximum collection efficiency. The second function of the CLR is to control and limit current.
Power control in a precipitator is achieved by silicon controlled rectifiers (SCRs). Two SCRs are connected in an inverse parallel arrangement in series between the power source and the precipitator high voltage transformer. The power source is an alternating current (AC) sinusoidal wave form whose value is zero at the beginning and end of every half cycle, and is a positive value during one half cycle and a negative value during the next half cycle. For a power source with a 60 Hz. frequency, this would occur every 8.33 milliseconds. (10 milliseconds for a 50 Hz. power source). Only one SCR conducts at a time on alternate half cycles. The automatic voltage control provides gating such that the appropriate SCR may be switched on at the same point during the half cycle to provide power control. The SCR remains switched on or in conduction until the current passing through the SCR falls below a specified value for the device. The cycle is then repeated for the next half cycle and the opposite SCR. The SCRs cannot be switched off by the automatic voltage control. If the precipitator spark level is reached with no control of current to the precipitator, equipment damage can occur. The CLR provides a means of controlling and limiting the current flow to the precipitator until the conducting SCR switches off at the end of the half cycle.
Because of its critical role in maximizing electrostatic precipitator performance, it is vital that the CLR be properly sized. In the prior art, the CLR is sized at 30%.50% of the impedance of the transformer/rectifier (T/R) set. This calculation results in a rough estimate of the appropriate CLR size for a given application. The actual electrical efficiency is subjectively measured by viewing the shape and duration of the wave form of the secondary current with an oscilloscope and estimating the fractional conduction. The CLR is then adjusted by trial and error in an attempt to obtain the desired fractional conduction and, thereby, collection efficiency. Fractional conduction and other methods used to size CLRs in the prior art have been crude and inaccurate, allowing for operational inefficiency and equipment damage including blown fuses, equipment failure and inefficient performance from other components of the system.
The production output of many industries may be limited by the amount of pollution discharged. The government sets limits on the amount of pollution a facility may generate and discharge. In the event this limit is exceeded, a facility is subject to fines and temporary or permanent shut-down. Therefore, in terms of profitability, it is imperative that the electrostatic precipitator operate at its highest efficiency, and in the event of a malfunction, minimizing down time is a high priority.
The prior art requires time consuming calculations to determine initial operation settings for precipitator controls. In the event of a malfunction or fault, determining the exact problem and repairing or replacing the faulty component is time consuming and often requires disassembling of much of the precipitator or its controls. These limitations of the prior art all lead to operation inefficiency, equipment damage, inadequate performance and increased pollution emissions.